The present invention relates to an apparatus for detecting a collision of vehicles, specifically for use in a vehicular air-bag system.
FIG. 6 is a block diagram of a conventional apparatus for detecting a collision of vehicles of the type disclosed in, for instance, Japanese Patent Laid-Open No. 241467/1988. In FIG. 6, a piezo-electric element 1 is as an acceleration sensor, and a signal processing circuit 2 is on the output side of the piezo-electric element 1. The signal processing circuit 2 comprises an impedance converter 3 connected to the output terminal la of the piezo-electric element 1, an integrator 4 on the output side thereof, a comparator 5 for outputting a high-level signal when the output voltage of the integrator 4 exceeds a set voltage, and a transistor 6 which is turned on at the high-level signal from the comparator 5.
The transistor 6 is provided in an ignition circuit in which a detonator 7 for operating an air bag is connected to a battery 8. When a shock exceeding a predetermined value is applied to the piezo-electric element 1 at the time of collision, the output voltage generated from the piezo-electric element 1 is applied via the impedance convertor 3, the integrator 4 and the comparator 5 to the transistor 6, which is instantly turned on and the detonator 7 is ignited thereby. A mechanical acceleration sensor 9 is provided for preventing an explosion in error, this sensor having a contact with the ignition circuit, and a fail-safe relay 10 is provided to the ignition circuit.
A diagnostic pulse generator 11 is connected to the input terminal 1b of the piezo-electric element 1 to give the counter piezo-electric effect thereof and a diagnostic circuit 12 is provided at the output of the signal processing circuit 2, that is, at the emitter of the transistor 6 to detect the on/off state of the emitter. The diagnostic circuit 12 controls the on/off state of the fail-safe relay 10 and also controls a warning lamp for example 13 provided according to needs.
For instance, on receiving a vehicle speed signal, a cranking signal and a parking brake signal, the diagnostic pulse generator 11 confirms cranking time during the stopping of a vehicle and generates a diagnostic pulse. When the diagnostic pulse is applied to the input terminal 1b of the piezo-electric element 1, output voltage due to the reverse piezo-electric effect is generated from the output terminal 1a. The period and voltage of the diagnostic pulse is set so that what is higher than the set voltage V1 of the comparator 5 is supplied from the integrator 4. Consequently, the transistor 6 is turned on when the diagnostic pulse is generated unless there is a malfunction of the piezo-electric element 1 and the signal processing circuit 2, whereby the voltage is applied from the battery 8 to the diagnostic-circuit 12. When no voltage is applied to the diagnostic circuit 12 within a predetermined lapse of time after the generation of the diagnostic pulse, the diagnostic circuit 12 decides the presence of trouble and turns the fail-safe relay 10 off and then activates an alarm 13. Since the diagnostic pulse is produced during the stopping of a vehicle, the contact of the mechanical acceleration sensor 9 remains off and the detonator 7 is prevented from ignition.
The conventional apparatus for detecting a collision of vehicles is thus constructed and it involves a risk of explosion in error in case an impulse is applied thereto by mistake in such a state that the power supply has been kept on while the apparatus is being repaired.